Utilizing a highly sensitive in vitro plaque forming cell assay which was developed in this laboratory to study human B lymphocyte function, we have delineated several of the complex mechanisms of immunoregulation in normal immunologic reactivity and in diseases characterized by abberancies of immunoregulation. We have described the mechanisms and events associated with B cell triggering and subsequent differentiation and production of antibody. We have characterized the kinetics, mechanisms, and subpopulations involved in the regulation of B cell function in the normal immune response. In addition, we have demonstrated that normal immune reactivity is the net result of a balance between positive and negative immunoregulatory influences mediated directly or indirectly by distinct subpopulations of cells. In diseases characterized by abnormalities of immunoregulation, we have related several abnormalities of immune function directly or indirectly to the increase or deficiency of various regulatory cell subpopulations, abnormalities in their functional capabilities, as well as to the state of their activation. The differential sensitivity of these various parameters to a variety of clinically relevant manipulations such as corticosteroid therapy has been described.